Multiband OFDM-UWB (Orthogonal Frequency Division Multiplexing Ultra Wideband) has been expected as a system of short-distance high-speed communication. In a configuration of a transmitter of this system, a direct conversion system, which modulates baseband signals to RF signals directly without converting into intermediate frequencies once, is used. Because a direct conversion transmitter can be realized by a simple configuration, miniaturization can be expected. On the other hand, as shown in FIG. 1(a), it is known that a phenomenon called LO leakage, that is, signals of local oscillator (hereinafter, referred to as LO) leak to the antenna side through a transmission mixer, occurs. Here, FIG. 1A shows an example of a block diagram of a transmitter in relation to the present invention, FIG. 1B shows another example of a block diagram of the transmitter in relation to the present invention, and FIG. 1C shows an example of a block diagram of a receiver in relation to the present invention.
This LO leakage is generally canceled by adding a DC signal which generates an LO signal with a reversed phase to the LO leakage to the baseband input of a mixer by means of a DAC (D/A converter) or the like. As shown in FIG. 1B, an example of correcting LO leakage is disclosed which includes detecting a carrier leakage amount in a spectrum by using an RF amplitude detector and working with a signal generator (in patent document 1, for example). In this multiband OFDM-UWB system, each band has a 500 MHz bandwidth as shown in FIG. 3A, and a UWB signal hops in turn from band to band per each symbol of 1401. As shown in FIG. 3B, the transmitter needs to make the center frequency hop at high speed in approximately 300 ns cycles. FIG. 3A is a figure showing the frequency characteristics at the time of transmission in the transmitter in relation to the present invention, and FIG. 3B is a figure showing the characteristics of the local frequency in the transmitter in relation to the present invention (for example, refer to patent document 2).
An example of a transmitter which transmits such high-speed hopping signals is proposed in patent document 3. In the related transmitter disclosed in the patent document 3, after switching the frequency of the hopping local signal, the transmitter makes its amplitude increase gradually and after reaching a peak, makes it flat for a while. After that, the transmitter makes the amplitude of the LO signal decrease gradually, and after the amplitude has become zero, it gives the envelope control to switch the frequency to the next one, so the transmitter suppresses the side tone that arises around OFDM signals.
Further, an example of a transmitter, which prevents frequencies other than a required frequency from mixing by stopping frequency dividers other than a frequency divider which generates the required frequency in a multiband generator which generates a plurality of frequencies, is disclosed (for example, refer to patent document 4).
In addition, technologies in relation to the above-mentioned ones are disclosed in patent documents 5-10. A system described in the patent document 5 is a system to calibrate LO leakage and sideband image for an RF transmitter, and it includes a calibration control logic unit and a sensor of an LO leakage and sideband image. The sensor of LO leakage and sideband image senses an LO leakage amount and a sideband image amount on the output of the RF transmitter. The calibration control logic unit receives the sensed LO leakage amount and the sensed sideband image amount and carries out calibration control for the RF transmitter by performing calibration for suppressing LO leakage first, next, performing calibration for suppressing the sideband image.
According to this system, LO leakage is calibrated on the output of the RF transmitter in the state that test tone is being inputted, and the minimum LO leakage is determined. After that, the sideband image is calibrated on the output of the RF transmitter in the state that the test tone is being inputted, and the minimum sideband image is determined. The operational value of the RF transmitter which should be used in a usual operation mode of the RF transmitter is stored in a memory based on the detected minimum LO leakage and the detected minimum sideband image.
A radio communication apparatus described in the patent document 6 is a radio communication apparatus which receives a communication signal hopping among a plurality of frequency bands, and is provided with a frequency conversion part, a high-pass filter part and a receiving processing part. The frequency conversion part performs frequency conversion by multiplying a received communication signal by the local signal with a hopping frequency. The high-pass filter part has capacitors in parallel corresponding to each frequency band for frequency hopping, and switches a connection among the capacitors synchronizing with the frequency hopping. The receiving processing part performs receiving processing of receiving signals passing through the high-pass filter part.
According to this radio communication apparatus, it is supposed that an excellent radio communication apparatus can be provided, which can suitably perform receiving processing by converting the frequency of multiband OFDMUWB signals of which frequency is switched among wideband.
A method described in the patent document 7 fits into an ultra wideband (UWB) receiver, and the UWB receiver receives a series of Multiband Orthogonal Frequency Division Multiplexing (MBOFDM) symbol signals transmitted from a UWB transmitter. Each MBOFDM symbol signal includes a zero padded prefix, a data signal and a guard interval. Time of the zero padded prefix is T1, and time of the guard interval is T2, and T1 is larger than T2, and the total time of the MBOFDM symbol signal is T3. The method described in the patent document 7 is a method for defining a symbol timing window and capturing a signal by the following two stages. Time is delayed by only presetting time T from a starting point of the zero padded prefix, it is set at a starting point of the symbol timing window, time of the symbol timing window is set to T3, and a plurality of receiving symbol signals are received through the symbol timing window. Each receiving symbol signal includes a prefix signal, a received data signal and a guard signal. Time of the prefix signal is T1−T, and time of the guard signal is T2+T, and there is a relation of T≦T1−T2 among T, T1 and T2. The received data signal and the guard signal are captured, the guard signal is added to a head part of the received data signal, and the added received data signal is outputted.
According to this method, the starting point of the symbol timing window is changed. The starting point of the pre-padded prefix of 60.6 ns is changed to a starting point of the guard interval of 9.5 ns. The received data signal of 242.4 ns is captured after 9.5 ns passing, and then Fast Fourier Transform (FFT) is carried out. In the UWB receiver, it is supposed that after the channel effect arises, it is copied in a head part of the received data signal, and orthogonality of the received data signal is kept by that means, and consequently FFT is carried out correctly.
A frequency hopping modulated wave receiving circuit described in the patent document 8 is provided with a synthesizer for generating local oscillation signals and an intermediate frequency amplifier. The synthesizer for generating local oscillation signals can switch over the oscillation frequency by a frequency hopping control signal, and it is locked to the frequency after switching over. The intermediate frequency amplifier is controlled by an AGC control signal. A holding circuit, which is controlled by the frequency hopping control signal and a lock signal from a frequency synthesizer and keeps the AGC control signal fixed at the time of switching over frequency, is provided in the AGC control circuit of the intermediate frequency amplifier.
According to this frequency hopping modulated wave receiving circuit, the AGC control signal at the time of switching over frequency can be kept fixed. Therefore, it is supposed that the output of the intermediate frequency amplifier at the time of switching over frequency becomes stable, and degradation of clearness of voice output caused by gain fluctuation of the intermediate frequency amplifier can be prevented by that means.
A transmitter in the frequency hopping spread spectrum communication described in the patent document 9 is provided with a plurality of frequency generators, a switching means and a modulation means. The frequency generator changes a frequency in turn with keeping the constant frequency during a fixed duration, with timing of changing the frequency being different in each other. The switching means switches over frequency signals outputted from a plurality of frequency generators at predetermined timing so that one of frequency signals may be derived in a stable state of the frequency change. The modulation means modulates the frequency signal derived by the switching means with a data signal.
According to this transmitter, while the output of the frequency generator selected by the switching means is modulated with data in order to put the data on, one of other frequency generators at least is changing its frequency. And in the stable state where the change has ended, the output of this frequency generator is selected by the switching means. It is supposed that by arranging in this way, because the transition duration of the frequency change in each frequency generator is not related to formation of a transmission signal, the transmission speed of signals can be increased.
A clock signal reproducing circuit described in the patent document 10 is a clock signal reproducing circuit which reproduces a clock signal from a receiving signal in the frequency hopping communication. This clock signal reproducing circuit includes a removal means which removes a carrier frequency switching timing part in a receiving signal from the receiving signal and a clock reproduction means which reproduces the clock signal from the receiving signal after this removal.
According to this clock signal reproducing circuit, the carrier frequency switching timing part in the receiving signal is removed from the receiving signal. Specifically, a synchronized signal which is synchronized with the carrier frequency switching timing is generated from the receiving signal, a switch is controlled according to this generated synchronized signal and the carrier frequency switching timing part is removed from the receiving signal. It is supposed that the analog switch is used for this switch.    Patent document 1: Japanese Patent Application Laid-Open No. Hei 10-136048    Patent document 2: U.S. Patent Application Publication No. 2004/0047285    Patent document 3: Japanese Patent Application Laid-Open No. 2005-286509    Patent document 4: Japanese Patent Application Laid-Open No. 2005-175698    Patent document 5: Japanese Patent Application Laid-Open No. 2006-25426    Patent document 6: Japanese Patent Application Laid-Open No. 2006-203686    Patent document 7: Japanese Patent Application Laid-Open No. 2006-211034    Patent document 8: Japanese Patent Application Laid-Open No. Hei 1-202031    Patent document 9: Japanese Patent Application Laid-Open No. Hei 8-46549    Patent document 10: Japanese Patent Application Laid-Open No. Hei 8-265219